The present invention relates to the measurement of torque generated in a drive shaft. More particularly, it concerns the non-contacting measurement of such torque using magnetised transducers and seeks to compensate for, eliminate or avoid the effects of interfering magnetic fields.
There have been prior proposals to use magnetised transducer elements for torque measurement, the transducer elements being a ring attached to a torqued shaft or the shaft itself. In this connection reference is made to U.S. Pat. Nos. 5,351,555, 5,465,627 and 5,520,059 and to published PCT Applications WO99/21150, WO99/21151 and WO99/56099. In these specifications the ring or shaft is of magnetoelastic material circumferentially magnetised, that is the magnetisation forms a closed loop around the shaft. While such transducer elements are usable in the practice of this invention, other patterns of magnetisation are usable and do not necessarily rely on magnetoelasticity, and other shapes of transducer element may be employed. One other pattern of magnetisation which may be employed in the practice of this invention longitudinal magnetisation of the transducer region. One form of longitudinal magnetisation is disclosed in International patent application PCT/GB00/03119 filed 14th Aug. 2000 and published under the number WO/01/13081 A1.
It is a feature of transducers systems employing magnetised transducer elements of the kind outlined above, that the torque dependent field component provided by the transducer element can be sensed by one or more sensors adjacent to but not in contact with the transducer elements. Non-contacting sensor arrangements are of particular value in torque measurement on rotating shafts.
The above techniques are based on magnetic principles and therefore can be affected by other interfering magnetic fields, like the earth""s magnetic field or fields generated by electric motors for example. In some environments where it is desirable to measure shaft torque, very strong magnetic fields may be present, particularly in the longitudinal axis of the sensing system. A typical application of this nature is the extended axis of an electric motor having a shaft projecting from the motor.
The present invention is predicated on a number of different approaches. A first may be broadly expressed as compensating or counteracting an interfering magnetic field. A second may be broadly expressed as a selective signal approach, particularly by introducing a frequency selective element into the torque-dependent magnetic flux to be measured that enables it to be distinguished from signals due to an interfering field. A third approach is to turn the xe2x80x9cinterferingxe2x80x9d magnetic field to use and employ it as a source field from which to obtain a torque-dependent component. A fourth approach is a new way of measuring torque to which a frequency selective element may be applied. It is possible to use combinations of these approaches, particularly in combining the first approach with the second or third.
One implementation of the present invention according to the first approach above-mentioned provides a torque transducer for measuring torque in a rotating shaft of the kind having a transducer region in which a magnetic transducer field is established and at least one non-contacting sensor adjacent the transducer region to develop a torque-dependent signal, wherein in operation the shaft is subject to longitudinal flux generated by means external to the transducer region, characterised by means magnetically coupled to said shaft to generate a compensating flux to counteract said longitudinal flux at the transducer region.
Preferably, the means coupled to the shaft for generating the compensating flux comprises at least one current-carrying coil about the shaft. It may comprise a pair of axially spaced coils between which the transducer region is situate. In the alternative or additionally, a magnetic structure may also be provided which has poles axially spaced along the shaft and at least one coil is wound about said magnetic structure.
An implementation of the invention according to the third approach above-mentioned provides a torque transducer for measuring the torque in a rotating shaft which, in operation, has a longitudinal field extending therealong, wherein at least one sensor is placed in non-contacting fashion adjacent a portion of the shaft to sense and provide a signal dependent on a transverse component of flux arising from the longitudinal flux in response to the torque in the shaft. More specifically a transverse component is transverse to the axis of rotation and at the surface of the shaft portion is usually detected as a component in the circumferential or tangential direction. In the preferred embodiment, at least one further non-contacting sensor is mounted to sense the longitudinal flux to provide a reference signal dependent thereon against which to measure the transverse component for use in obtaining a value for the torque in the shaft.
In yet another implementation of the invention, this time in accord with the second, selective signal approach above-mentioned, a torque transducer for measuring the torque in a rotating shaft includes a portion or region of the shaft which acts as a transducer element and which is disposed between a pair of coils encircling the shaft and connected to induce a longitudinal magnetic field through the transducer region upon energisation of the coils. The coils are connected to an AC source, preferably a pulsed source, operating at a selected frequency so that the transducer region is subject to a magnetic field of alternating polarity. A sensor arrangement is responsive to a torque-dependent component of the alternating magnetic field and provides an AC output processed in a frequency-selective manner linked to the source frequency to extract the wanted component from any other noise (DC or AC) that may be present. The frequency-selective processing may be by way of a hardware or software implemented filter operating at the selected frequency linked with the AC source to synchronize the filter frequency to the source frequency. A synchronous detection scheme can be used detecting the sensor output signal with the aid of the AC source output to provide an inherent filtering operation.
According to another implementation, a transducer assembly for measuring, preferably in a non-contacting fashion, torque in a rotating shaft, comprises an erase head for cleaning a zone of the shaft as it rotates, a write head downstream of the erase head in the direction of rotation to write a magnetic track onto the cleaned zone, said track having a given width, a pair of read heads spaced in an axial direction to respond to the magnetic track, said read heads being disposed on, toward or adjacent opposite sides of the track to generate respective signals, and differential means responsive to said respective signals to provide a signal dependent on torque in the shaft. It is preferred to energise the write head with an AC signal, preferably a pulsed signal, to detect the AC outputs of the read heads derived from the AC modulated track. The detection can be done in a frequency-selective manner to enhance discrimination from other signal fields that may be present. It is preferred that the write head be oriented with the head gap in the circumferential or tangential direction.
Aspects and features of this invention are set forth in the claims following this description.